1. Field of the Invention
The invention relates to a power converting apparatus, and more particularly to a switching power converting apparatus.
2. Description of the Related Art
Referring to FIG. 1, a conventional flyback switching power converting apparatus is shown to include a voltage conversion module 9, and a switching controller 8. The voltage conversion module 9 includes a transformer 90, a transistor 91, a diode 92, and a capacitor 93. The transformer 90 includes a primary winding for receiving an input voltage (Vin) and a secondary winding. The transistor 91 is coupled between the primary winding of the transformer 90 and ground, and is controlled by a pulse width modulation (PWM) signal to conduct or non-conduct. The diode 92 is coupled to the secondary winding of the transformer 90 at its anode. The voltage conversion module 9 is operable based on the PWM signal to convert the input voltage (Vin) into an output voltage (Vout) across the capacitor 93. The larger a duty cycle of the PWM signal is, the larger a secondary side current (I2) flowing through the secondary winding of the transformer 90 will be. The switching controller 8 is operable to generate the PWM signal based on a reference voltage (Vref) and the output voltage (Vout) so that the smaller the output voltage (Vout) is, the larger the duty cycle of the PWM signal will be.
Initially, the output voltage (Vout) is not yet raised. Therefore, the duty cycle of the PWM signal generated by the switching controller 8 is maximum. In this case, referring to FIG. 2, a primary side current (I1) flowing through the primary winding of the transformer 90 and the transistor 91 becomes an inrush current at a time point (t1) that may result in damage to the transistor 91.